Automatic frequency control system



w. w. HANSEN ETAL 2,414,100

AUTOMATIC FREQUENCY CONTROL SYSTEM "Jan. 14, 1947.

Original Filed Jan. 16, 1942 l.F. mpuruzn FREQUENCY mscnmmnon l 2 ELr,-r O

UTILIZKTION cmcul'r INVENTORS W. W. HANSE N BY E: L. GINZTON ATTORNEYPatented Jan. 14, 1947 AUTOMATIC FREQUENCY CONTROL SYST William W.Hansen and Edward L. Ginzton, Garden City, N. Y., assignors, by mesneassignments, to Board of Trustees of the Leland Stanford JuniorUniversity, Stanford University, Calif.

Original application January 16, 1942, Serial No. 426,986. Divided andthis application September 25, 1943, Serial No. 503,759

Claims. 1

The present invention relates. generally, to ultra-high-frequencysystems operating at frequencies of the order of or 10 cycles per secondand, more specifically, to superheterodyne receiving systems operatingat such frequencies. The present application is a division ofapplication Serial No. 426.986. filed January 16. 1942, and is acontinuation-in-part of U. S. Patent No. 2.294.942 for Fixed frequencydifference and stabilization system, granted September 8, 1942.

In ultra-hiah-frequency receiving svstems of the superheterodvne tyne.difficulty is experienced in maintaining prop r operation due to theinherent tendency of local oscillators operating at such high freuencies to drift appreciably in frequency. whereby the intermediate freuency produced by beating the received fre uency with the localoscillator frequency may drift so widely as to pass beyond the frequencyacce tance range of the intermediate-frequency amplifiers or otherintermediate-frequency stages of the system. According to the presentinvention, and to prevent this harmful effect. improved a paratu isprovided to maintain the local oscillator frequency at a fixed frequencydifference with respect to the received wave. In particular. the ocaloscillator utilized is of the velocity-modulation type. such asdescribed in U. S. Patent No. 2,242.2'75. granted May 20, 1941. The outut of this ultra-highfrecuency local oscillator is combined with thereceived wave in an ultra-hi h-freouency mixer. or first detector. alsoof the velocity-modulation type. ,The intermediate frequency output ofthis ult a-hi h-freouency mixer. after suitab e amplification of anyconvenient type. is supplied to a frequency discriminator ofconventional tvpe which is adapted. as is well known. to produce zerovoltage output when the input frequency is equal to the tuned frequencyof the discriminator. which is made to correspond to the desiredintermediate frequency. The discriminator also produces an outputvoltage of predetermined polarity when the input frequency is greaterthan the desired intermediate fre uency. and an output voltage ofopposite polarity when the input frequency is lower than the desiredvalue.

This output of the frequenc discriminator is then utilized to controlthe local oscillator frequency by controlling one of the freuency-determining characteristics of this oscillator. In the presentinstance, this is illustrated as controlling the local oscillatorfrequency by controlling the electron-beam-accelerating voltage in thevelocity-modulation type of local oscillator. It is to be noted that thepresent system is also equally adapted to maintain a controlledultrareceived wave in the superheterodyne system discussed above.

Accordingly, it is an object of the present invention to provide animproved frequency-stabilized ultra-high-frequency superheterodynereceiving system,

It is another object of the present invention to provide improvedultra-high-frequency apparatus for maintaining a controlled frequencysource at a fixed frequency difference with respect to a referencefrequency.

It is still another object of the present invention to maintain theoutput fre uency of an ultra-hi h-frequency velocit -modulationoscillator at a fixed frequency difference with res ect to a referencefrequency. by contro ling the electron-beam-accelerating voltage of thecontrolled oscil ator.

Other objects and advantages of the present invention will becomeapparent from the s ecification and attached drawing. wherein the singlefigure illustrates a schematic wiring diagram of a preferred embodimentof our invention.

Referrin to the single figure of the drawing, the controlled localoscillator is shown in the form of a velocity-modulationoscillator-buffer 96. having three cavity resonators 3|. 32 and 33; eachof which has a pair of electron-permeable grids defining a gap. Anelectron beam derived; from cathode 34 is adapted to be projectedsuccessively through these resonator gaps by meansof the acceleratin batery-or other unidirectional volta e source I01. The ga s of resonators3| and 3?. are separated by a field-free drift space 35, while the gapsof resonators 32 and 33 may be made to adjoin one another closely.

It is to be understood that resonators 3!. 32, and 33 are all tuned(that is, have their resonant frequencies suitably adiusted by anywell-known means) to the desired operating frequency for oscillator 96.r

Inoperation, the electron beam from cathode 34 passes first through-thegap of resonator 3| and is therein velocity-modulated by any ultra.-high-frequency electric field existing at this gap by virtue ofoscillations within the resonator 3|. Such velocity-modulation of thebeam is converted into bunching or grouping of the electrons of thebeams by their subsequent traversing of drift space 35. The bunchedelectron beam then 1 355 5 across the gap of resonator 32 and deliv- 3ers ultra-high-frequency energy to this resonator 32.

In this manner the electron beam excites an ultra-high-frequencyelectromagnetic field within resonator 32. Some of this energy is fedback to resonator 3| by way of the feed-back coupling loop ortransmission line 36 to sustain osci lations within resonators 3| and 32by regenerative action. In this way the bunching of the electron beam isself-sustained. The bunched beam also passes across the gap of bufferresonator 33 and thereby excites this resonator to oscillation also. Theoscillatory energy of resonator 33, indicated as being of the frequencyis, may then be extracted from resonator 33 by a suitable couplingtransmission line 2|. Since resonator 33 is coupled to the oscillatingsystem of resonators 3| and 32 only by the electron beam, the externalcircuit coupled to resonator 33 cannot react on the oscil ator to varits frequency.

Also shown in the figure is an ultra-highfrequency velocity-modulationmixer-detector l8 which comprises a pair of cavity resonators 4! and 42separated by a field-free dr ft space 63. Resonator 4| is coupled to areceiving antenna 2 by means of the coupling line M and is tuned to thefrequency of the received wave, indicated as f1. Resonator 42 is coupledto resonator 33 of oscillator 96 by way of transmission line 2| and istuned to the frequency f2.

An electron beam is projected successively through the gaps between thegrids of resonators 4| and 42 from a cathode 45 by means of a suitableaccelerating voltage source ifi. By virtue of the fields excited withinresonators 5| and 42 by antenna 2 and oscillator 95, the electron beamis bunched both at the frequency fl and the frequency is. A detectorelectrode 41, maintained at a potential near that of cathode 55 andpreferably negative with respect to the ground potential of the grids ofthe resonators 4| and 42, supplies a retarding field for the bunchedbeam. By adjusting the potential of electrode 41 to a value such thatall electrons of less than average velocity are repelled thereby, theelectron beam current passing through the grids of the detectorelectrode 41 and collected by a suitable target electrode 48 will have acomponent of frequency f1--f2, corresponding to the beat frequencybetween the frequencies f1 and f2.

A tuned circuit 22 tuned to this beat or intermediate frequency is thenconnected between the target electrode 48 and ground, and serves to passonly the intermediate-frequency wave to the intermediate-frequencyamplifier 38, which may be of any conventional type. The output ofamplifier 38 is then supplied to the conventional frequencydiscriminator I04 which is also tuned to this same intermediatefrequency. The output of the discriminator I04 is then supplied to thecontrol grid of a tube Hi6 connected in series with the acceleratingvoltage source lfl'l for oscillator 96.

The output frequency of a velocity-modulation type of oscillator such as$6 has been found to depend upon the electronbearn-accelerating voltage.By controlling the electron beam voltage by tube I06, in response to theoutput of discriminator I04, the output frequency of oscillator 96 maybe adjusted to the proper value to maintain the intermediate frequencyclosely to the desired value f1-,f2 at which the discriminator HM istuned.

In this way any variation in intermediate frequency, such as due todrifting of the oscillator 96 or to changing of the received frequencyi1, is automatically compensated by varying the accelerating voltage ofthe electron beam of oscillator 96 to maintain the difference betweenthe two frequencies f1 and is at the value determined by thediscriminator I04.

The intermediate-frequency wave derived from circuit 22 may also besupplied to any desired type of utilization circuit 50, such as thatdisclosed more fully with respect to Fig. 2 of parent application SerialNo. 426,986.

It is to be understood that antenna ||2 may be replaced by any suitabletype of source of reference frequency with respect to wich the outputfrequency of oscillator 96 is to be maintained at the fixed frequencydifference determined by the tuned frequency of discriminator H36.

Since many changes could be made in the above consrtuction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. High frequency superheterodyne receiver apparatus comprising a highfrequency oscillator having a pair of coupled cavity resonators, abuffer cavity resonator, and means including a source of beamaccelerating voltage for projecting an electron beam successivelythrough said resonators; a mixer-detector having a pair of cavityresonators, an output electrode, and means for projecting a beam ofelectrons through said resonators to said electrode; a source ofreceived Wave; means for coupling said source to one of saidmixer-detector resonators; means for coupling said buffer resonator tothe other of said mixer-detector resonators; a frequency discriminatortuned to a desired frequency difference between the output frequency ofsaid oscillator and the frequency of said source; means for connectingsaid discriminator to said electrode; and means responsive to the outputof said discriminator for controlling said beam accelerating voltage, tomaintain said oscillator frequency at the value required to produce saiddesired frequency difference, said last-named means comprising a triodehaving its anodecathode circuit in series with said accelerating voltagesource and its grid energized by said discriminator output.

2. High frequency apparatus comprising a high frequenc oscillator havingan output cavity resonator and means for exciting oscillations in saidresonator, said exciting means including a cathode and a source ofelectron-accelerating voltage for projecting an electron beam throughsaid resonator; a mixer-detector having a pair of cavity resonators andmeans for projecting a beam of electrons through said resonators toprovide an output corresponding to the beat frequency of the wavessupplied to said resonators; means for coupling a source of referencefrequency to one of said mixer-detector resonators; means for couplingsaid output resonator to the other of said mixer-detector resonators, afrequency discriminator tuned to a desired frequency difference betweenthe output frequency of said oscillator and said reference frequency;means for energizing said discriminator from the output of saidmixer-detector; and means rev sponsive to the output of saiddiscriminator for controlling said accelerating voltage source tomaintain said oscillator frequency at said desired frequency differencewith respect to said reference frequency, said last-named meansincluding an electron discharge device having a cathode, a controlelectrode and an anode, said cathode and anode being connected in serieswith said source, and said control electrode being connected to saiddiscriminator output for controlling the conductivity of said device inaccordance with said discriminator output.

3. High frequency apparatus comprising a high frequency oscillatorhaving a pair of coupled cavity resonators, a buffer cavity resonator,and means including a source of beam-accelerating voltage for projectingan electron beam successively through said resonators; a source ofreference frequency; means for obtaining the beat frequency between saidreference frequency and the output frequency of said oscillator derivedfrom said buffer resonator; a frequency discriminator tuned to a desiredfrequency difference between said oscillator frequency and saidreference frequency; means for actuating said discriminator by said beatfrequency; and means responsive to the output of said discriminator forcontrolling said beam-accelerating voltage, to maintain said oscillatorfrequency at a value producing said desired frequency difference, saidlast-named means comprising a triode having its anode-cathode circuit inseries with said accelerating voltage source and its control gridenergized by said discriminator output.

4. High frequency apparatus comprising a high frequency oscillatorhaving a pair of coupled cavity resonators and a buffer cavityresonator, and means including a source of beam-accelerating voltage forprojecting an electron beam successively through said resonators; asource of reference frequency; means coupled to said buffer resonatorand to said source for producing a beat frequency between said referencefrequency and the output frequency of said oscillator; and meansresponsive to deviation of said beat frequency from a desired value forcontrolling said oscillator frequency to maintain said desired beatfrequency value, said last-named means including means for producing asignal corresponding to said frequency deviation, and an electrondischarge device connected to said source and having a control electrodeconnected to be controlled by said signal, whereby saidbeam-accelerating voltage is controlled in accordance with saidfrequency deviation.

5. High frequency apparatus comprising a high frequency oscillatorhaving an output cavity resonator, and means for projecting an electronbeam through said resonator to excite oscillations therein, saidlast-named means including a source of electron-beam-acceleratingvoltage; a source of reference frequency; means for producing a beatfrequency between said reference frequency and the output frequency ofsaid oscillator; and means responsive to deviation of said beatfrequency from a desired value for varying said oscillator frequency tomaintain said desired beat frequency value, said last-named meansincluding means responsive to said frequency deviation for producing acontrol signal, an electron discharge device connected to saidaccelerating voltage source and having a control electrode, and meansenergizing said control electrode by said signal, whereby saidaccelerating voltage is controlled in response .to said frequencydeviation.

WIILIAM W. HANSEN. EDWARD L. GINZTON.

